No disease in the home orchard produces more visible, dramatic damage in a shorter time than fire blight. A pear tree branch that looks healthy on Monday can appear to have been scorched with a torch by Friday. The name is well chosen.
Fire blight is caused by the bacterium Erwinia amylovora, and per Cornell Cooperative Extension, it is one of the most destructive bacterial diseases of apples and pears in North America. The disease can kill individual branches, entire trees, and in severe outbreaks can devastate commercial orchards.
On Long Island, fire blight is a routine concern for anyone growing pear or apple trees. Wet spring weather during bloom — which is typical from late April through mid-May in zone 7a — creates ideal infection conditions. The disease shows up as blossoms and shoot tips collapse and blacken in late May and early June.
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What fire blight is
Erwinia amylovora is a bacterial pathogen native to North America. Per UC IPM, it was first described scientifically in 1780 on pear trees in the Hudson Valley of New York — not far from Long Island.
The bacterium overwinters in cankers on infected wood. Per Missouri Botanical Garden, in spring, bacteria multiply in the margins of these cankers and produce an ooze that insects, rain splash, and wind spread to open blossoms. Once inside the flower, the bacteria spread rapidly into the spur, then the branch, then down toward the trunk.
Per Cornell Cooperative Extension, infection requires warm temperatures (above 65°F, optimal near 80°F) and wetting events — rain, dew, or high humidity — during bloom. The combination of bloom timing, temperature, and moisture is used to calculate fire blight risk in commercial orchards.
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